1. Cite this: RSC Advances, 2013, 3,
18863
Wurtzite CuInS2: solution based one pot direct synthesis
and its doping studies with non-magnetic Ga3+
and
magnetic Fe3+
ions3
Received 9th April 2013,
Accepted 7th August 2013
DOI: 10.1039/c3ra41698d
www.rsc.org/advances
Meenakshi Gusain, Prashant Kumar and Rajamani Nagarajan*
Reactions of air stable copper-thiourea precursors [Cu(tu)3]Cl and [Cu2(tu)6]SO4?H2O with indium(III)
acetate (In(OAc)3?xH2O) and indium(III) sulfate (In2(SO4)3?xH2O) under refluxing in ethyleneglycol for 1–3.5
h yielded metastable, wurtzite (WZ) and zincblende (ZB) forms of CuInS2 (CIS). While the yields of CIS from
the reactions using In2(SO4)3 were quite high, reactions involving In(OAc)3 were sluggish producing low
yields. A flower like morphology has been observed in the FE-SEM image of the WZ-CIS with EDX analysis
yielding Cu : In : S ratio as 1.05 : 0.95 : 2.00. The SAED pattern of WZ-CIS recorded from the HR-TEM
images could very well be indexed in hexagonal symmetry. The room temperature Raman spectrum also
confirmed the formation of crystalline CIS. Solid WZ-CIS samples showed a band gap of 1.40 eV as revealed
by UV-Visible diffuse reflectance spectroscopic analysis. Doping of non-magnetic Ga3+
-ion and magnetic
Fe3+
-ion for the In3+
-ion in the WZ-CIS has been examined by reacting the sulfate salts of gallium and iron
with [Cu(tu)3]Cl and In2(SO4)3. FE-SEM-EDX and TEM-EDX analyses confirmed the presence of gallium and
iron in CIS samples. 3.5 at% gallium doped CIS sample showed WZ to be the major phase with few
reflections appearing due to the chalcopyrite phase in both the PXRD and TEM-SAED patterns. On doping
CIS with 20.8 at% of iron, the hexagonal symmetry of the CIS changed to either cubic (zincblende) or
tetragonal (chalcopyrite) depending on the experimental conditions. The tetragonal symmetry of the iron
doped CIS has also been verified from TEM-SAED patterns. Introduction of intermediate states in the
bandgap of CIS has been observed on doping with iron in the UV-visible diffuse reflectance spectrum with
the estimated band gap of 1.05 eV. From magnetization measurements at room temperature, Fe3+
-doped
CIS showed paramagnetic behavior with xg of 5.89 6 1026
emu/g. Also, they showed transitions between
the defect levels resulting in intense photoluminescence (PL) emission at 750 nm on excitation with l = 500
nm. The electron paramagnetic resonance (EPR) spectrum confirmed the presence of Fe3+
ions in the CIS
lattice exhibiting a broad signal at g = 1.90. The versatility of using this rapid and scalable synthetic
approach has been extended to produce orthorhombic AgInS2.
1. Introduction
Harvesting the naturally abundant solar energy using semi-
conductors for many useful applications is being pursued with
vigor.1
Among the I–III–VI semiconductors, CuInS2 (CIS) is
under intense investigation due to its excellent band gap (1.5
eV) match with the solar spectrum, high absorption coeffi-
cient, and good thermal, environmental and electrical stabi-
lity.2
The dual possibility of CuInS2 exhibiting n and p-type
semiconductivity by virtue of its tolerance to non-stoichiome-
try expands its list of applications as light emitting diodes and
non-linear optical devices.3
CIS is known to exist in three
polymorphic modifications, chalcopyrite (CH), zincblende (ZB)
and wurtzite (WZ). Among these, CH is the most common and
thermodynamically stable crystal structure in which the
cations Cu+
and In3+
are ordered in the cation sub-lattice sites
resulting in tetrahedral symmetry.4
Random distribution of
the cation leads to either a zincblende or a wurtzite structure
possessing cubic or hexagonal symmetry, respectively. Both ZB
and WZ forms are metastable at ambient temperature and can
only be stable above 1253 K and 1318 K respectively.4
Synthesis
of ZB and WZ polymorphs of CIS by solution routes is
considered to be advantageous for researchers to tune the
stoichiometry which in turn can modify the Fermi energy over
a wide range during the photovoltaic fabrication.5,6
Also, the
observation of WZ-CH polytypism, and intergrowth of CuAu
Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi
110007, India. E-mail: rnagarajan@chemistry.du.ac.in
3 Electronic supplementary information (ESI) available: PXRD pattern of the
product from the reaction of [Cu(tu)3]Cl with In(OAc)3 in ethyleneglycol for 3.5 h,
Le Bail fitting of the PXRD pattern WZ-CIS, its SEM and HR-TEM images, PXRD
pattern of the product from the reaction of [Cu4(tu)9](NO3)4?4H2O with In(OAc)3
and In2(SO4)3, PXRD pattern of CIS obtained from the reaction of [Cu(tu)3]Cl and
In2(SO4)3 under refluxing in ethanolamine, Le Bail fitting of the PXRD pattern
and HR-TEM image of Fe3+
-doped CuInS2 samples. See DOI: 10.1039/c3ra41698d
RSC Advances
PAPER
This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 18863–18871 | 18863

2. (CA) ordering with the CH phase necessitated further under-
standing to correlate the synthetic aspects with the structure of
CIS.7,8
A variety of synthetic approaches such as hydrothermal/
solvothermal techniques, single source precursor routes, and
hot injection methods in addition to solid phase synthesis
have been developed for making CIS both in bulk and in
nanosizes with varying morphologies.9–12
Despite the existence
of a huge number of reports, there still exists a requirement for
‘‘greener’’ solution based methods to synthesize metastable
WZ and ZB structures of CIS that are scalable, convenient, cost
effective, easy handling, one step and highly reproducible for
use in solar cells. The limited availability of single source
precursors and their tedious multistep synthetic procedure
have persuaded researchers to explore mixed precursors for
the synthesis of CIS.13
The fact that Cu(I) is a Soft acid and
In(III) is a Hard acid (as per the HSAB principle), thus differing
in their affinity for the Soft ligand sulfur, has been the major
synthetic difficulty encountered in solution routes employing
mixed precursors. This has successfully been overcome by the
usage of strongly coordinating solvents (thiols, amines) or a
mixture of high boiling point solvents to fabricate nano-
crystals of WZ and/or ZB phases in addition to CH-CIS.5–8
Though these organic reagents favored the formation of
kinetically controlled phases of CIS, their toxicity, noxious
odor and the excess amounts required are some of the major
concerns during the scaling-up processes. These factors
increase the cost and the energy input hampering the
production of large quantities of CIS on an industrial scale.
Generally, thermodynamically stable CH-CIS has resulted
from reactions of appropriate copper (CuCl2?2H2O,
Cu(NO3)2?xH2O), indium (InCl3?4H2O) and sulfur sources
(thiourea, thioacetamide, KSCN) in ethyleneglycol irrespective
of the duration of the reaction or under solvothermal
conditions.1d,14,15
The lower coordinating ability of ethylene-
glycol and its lower boiling point have been reasoned to be
major factors for not stabilizing the metastable WZ and ZB-CIS
phases. As ethyleneglycol is a ‘‘greener’’ solvent known to
promote homogenous mixing of the reactants leading to high
purity products within a short span of time,16
our motivation
is to realize metastable WZ and ZB-CIS in ethyleneglycol by
counterbalancing its lower coordinating ability with the use of
an appropriate molecular precursor consisting of Cu–S
coordination for reactions with air-stable indium(III) salts. It
is known that copper-thiourea (Cu-tu) precursors produce
Cu9S5 (Cu1.8S), Cu9S5 + CuS and CuS, on dissociation in
ethyleneglycol, depending on the anion of the precursor.16
Therefore, the Cu-tu precursors with chloride, sulfate and
nitrate counter anions were refluxed with indium(III) acetate
(In(OAc)3?xH2O) and indium(III) sulfate (In2(SO4)3?xH2O) inde-
pendently in ethyleneglycol. The band gap of CIS can be
engineered to higher and lower values by doping it with Ga3+
and Fe3+
ions respectively, thus making it an ideal combina-
tion for applications in the entire UV and visible regions. Also,
the introduction of these ions in the CIS lattice can have
additional advantages such as changing the carrier type and
magnetic functions as semiconductors for spintronics applica-
tions.17
Due to these reasons, the effect of substitution of non-
magnetic Ga3+
(ionic size of 0.47 Å) and magnetic Fe3+
(ionic
size of 0.49 Å high spin) for the In3+
(ionic size of 0.62 Å) on the
structure, optical and magnetic properties of CIS have been
examined.18
The phases were characterized by powder X-ray
diffraction (PXRD), field emission scanning electron micro-
scopy (FE-SEM), transmission electron microscopy (TEM),
selected area electron diffraction (SAED), Raman spectroscopy,
UV-visible diffuse reflectance spectroscopy, photolumines-
cence (PL) spectroscopy, atomic absorption spectroscopic
(AAS) analysis, electron paramagnetic resonance (EPR) spectro-
scopy techniques and magnetic measurements. Further, the
concept of using the metal-thiourea complexes with air stable
indium salts to stabilize metastable structures in other ternary
sulfides in ethyleneglycol medium has been extended to
obtain orthorhombic AgInS2.
2. Experimental
[Cu(tu)3]Cl, [Cu2(tu)6]SO4?H2O, [Cu4(tu)9](NO3)4?4H2O, and
[Ag2(tu)6]Cl2?2H2O were prepared, purified and characterized
following established procedures.19,20
In(OAc)3?xH2O (Sigma
Aldrich, 99.99%), In2(SO4)3?xH2O (Alfa aesar, 99.99%), and
In(NO3)3?xH2O (Sigma Aldrich, 99.999%) were used as received
without further purification. Ethyleneglycol (Merck, 99%) and
ethanolamine (Merck, ¢98%) were used as received.
[Cu(tu)3]Cl (0.6547 g, 2 mmol) was mixed well with
In(OAc)3?xH2O (0.5890 g, 2 mmol) in the solid state to which
50 ml of ethyleneglycol was added and refluxed for times
ranging between 1 h and 3.5 h. [Cu(tu)3]Cl (0.3273 g, 1 mmol)
and In2(SO4)3?xH2O (0.2589 g, 0.5 mmol) were reacted for 2.5
h. Similarly, [Cu2(tu)6]SO4?H2O (0.3489 g, 0.5 mmol) and
[Cu4(tu)9](NO3)4?4H2O (0.3148 g, 0.25 mmol) were refluxed
independently with In(OAc)3?xH2O (0.2919 g, 1 mmol) and
In2(SO4)3?xH2O (0.2589 g, 0.5 mmol) respectively in 50 ml of
ethyleneglycol for 2.5 h. For doping studies, Ga2(SO4)3?xH2O
(Sigma Aldrich, ¢99.99%) and Fe2(SO4)3?xH2O (Alfa Aesar,
Reagent Grade) were reacted with [Cu(tu)3]Cl and
In2(SO4)3?xH2O. For the synthesis of CuIn12xGaxS2 composi-
tions, 0.3273g (1 mmol) of [Cu(tu)3]Cl with 0.1942 g (0.375
mmol), 0.1295 g (0.25 mmol) of In2(SO4)3?xH2O and 0.0534 g
(0.125 mmol), 0.1069 g (0.25 mmol) of Ga2(SO4)3?xH2O
corresponding to the x values of 0.25, 0.50 were reacted
independently in 50 ml of ethyleneglycol for 2.5 h. Fe3+
-ion
doped CIS samples were synthesized by reacting 0.3273 g (1
mmol) of [Cu(tu)3]Cl with 0.099 g (0.25 mmol) of
Fe2(SO4)3?xH2O and 0.1295 g (0.25 mmol) of In2(SO4)3?xH2O
in ethyleneglycol for 2.5 h. Selected reactions in ethanolamine
were carried out in a similar fashion as in ethyleneglycol. For
the synthesis of AgInS2, [Ag2(tu)6]Cl2?2H2O (0.3837 g, 0.5
mmol) was reacted with In(NO3)3?xH2O (0.3008 g, 1 mmol) in
50 ml of ethyleneglycol for 1 h. The products, after refluxing,
were centrifuged and washed with doubly distilled water
18864 | RSC Adv., 2013, 3, 18863–18871 This journal is ß The Royal Society of Chemistry 2013
Paper RSC Advances

3. followed by ethanol and carbon disulfide. Samples were dried
naturally.
Characterization
PXRD patterns were collected using a high resolution D8
Discover Bruker diffractometer, equipped with a point detector
(scintillation counter), employing Cu-Ka radiation (l =
1.5418 Å) obtained through a Go¨bel mirror with a scan rate
of 1.0 s/step and step size 0.02u at 298 K and PANalytical’s
Empyrean diffractometer, equipped with a PIXcel-3D detector,
employing Cu-Ka radiation (l = 1.5418 Å) with a scan step size
of 0.01313u and 63.495 s/step. UV-Visible diffuse reflectance
spectra (DRS) of the samples were collected using a Perkin-
Elmer UV-Vis spectrophotometer (Model Lambda-35) using
BaSO4 as the reference. Raman spectra were recorded in
compact form using a Renishaw via Microscope system with a
diode laser (l = 785 nm and 514 nm). PL spectroscopy
measurements were carried out using a Horiba Jobin Yvon
Fluorolog 3 Spectrofluorometer at room temperature. The
morphology of the products was analyzed using a field
emission scanning electron microscope (FE-SEM), FEI
Quanta 200F equipped with EDX accessories. Transmission
electron microscopy (TEM) and selected area electron diffrac-
tion (SAED) were carried out using an FEI Technai G2
20
electron microscope operating at 200 kV. Magnetic measure-
ments were carried out at 300 K using a Vibrating Sample
Magnetometer (Micro sense EV9). The concentration of iron in
the samples was determined by subjecting the solution of
them in dilute nitric acid to atomic absorption spectroscopic
analysis employing a Shimadzu AA-6300 atomic absorption
spectrometer. EPR spectra were recorded using a Bruker EPR
spectrometer Biospin model A300. The signal channel was
calibrated before measurements using DPPH at 100 KHz
modulation frequency and with 6 G modulation amplitude in
the X-band 9.8 GHz.
3. Results and discussion
The reaction of [Cu(tu)3]Cl with In2(SO4)3 and In(OAc)3
independently, under refluxing conditions in ethyleneglycol
for a period of 1–3.5 h, has resulted in WZ-CIS as revealed by
their PXRD patterns (Fig. 1 and Fig. S1 ESI3). The lattice
constants, from the Le Bail fitting procedure in the P63mc
space group (# 186) are a = 3.923(5) Å and c = 6.455(4) Å (Fig. S2
ESI3). The average crystallite size of the WZ-CIS, estimated by
the Scherrer analysis, is in the range of 23–28 nm. Formation
of WZ-CIS has proved that the temperature of refluxing in
ethyleneglycol is sufficient enough to capture the metastable
phase. A flower like morphology of the WZ-CIS is noticed in its
FE-SEM image (Fig. S3 ESI3). The EDX spectrum along with
analysis of this sample shown in inset (a) of Fig. 1 yields a
Cu : In : S ratio of 1.05 : 0.95 : 2.00. Such a flower like
morphology has been reported for CH-CIS samples as well as
for Cu9S5 from [Cu(tu)3]Cl using ethyleneglycol.14b,15a,15c
The
selected area electron diffraction (SAED) pattern of WZ-CIS is
shown in inset (b) of Fig. 1. The diffraction spots correspond-
ing to the 100, 002, 101, 102, 110 and 112 planes observed in
the SAED pattern match well with the observed PXRD pattern.
The HR-TEM image of WZ-CIS is shown in Fig. S4 (ESI3). WZ-
CIS shows four bands at 256, 288, 302 and 344 cm21
in the
200–400 cm21
range in the Raman spectrum as shown in inset
(c) of Fig. 1. Of these, the band at 344 cm21
is high in intensity,
followed by the bands at 288 and 302 and 256 cm21
. Bands at
256 and 344 cm21
correspond to the B2 and E modes
originating from the antiphase vibrations of the In–S ions
and the Cu–S ions of CuInS2 respectively.21
The band at 288
cm21
is attributed to the A1 mode of the CuInS2 and the
shoulder at 302 cm21
indicates the presence of Cu–Au
ordering (A*-mode) in the sample.21
However, the estimated
ratio of these two modes is below 1.5 suggesting WZ to be the
dominant phase.21
While the formation of WZ-CIS is quite
rapid using In2(SO4)3 (within 1 h of refluxing), the reactions
involving In(OAc)3 are sluggish requiring refluxing times up to
3.5 h. This observation has reinforced the fact that the
formation of a WZ-phase in ethyleneglycol is a kinetically
controlled nucleation as proposed for reactions in high boiling
point solvents.5–8
A broad absorption ranging from 220 to 900
nm is observed in the UV-Visible diffuse reflectance spectrum
of WZ-CIS (Fig. 2(a)), from which the optical band gap of 1.40
eV has been estimated using the Kubelka–Munk function plot
(inset of Fig. 2 (a)). This value matches well with earlier
reports.21
A weak emission centred near 750 nm is noticed in
the PL spectrum on excitation with l = 500 nm, indicating the
transition between the defect levels present in the samples
(Fig. 2 (b)).14a,22
PXRD patterns of the products from the reaction of
[Cu2(tu)6]SO4?H2O with In2(SO4)3 and In(OAc)3 independently
are presented in Fig. 3 (a) and (b) respectively. Nanosized cubic
CIS (with an average crystallite size of 2.7 nm as estimated by
the Scherrer analysis) has emerged from the reaction of
[Cu2(tu)6]SO4?H2O with In2(SO4)3 (Fig. 3(a)). A mixed phase
containing CIS along with the unreacted In(OAc)3 is observed
Fig. 1 PXRD patterns of the products from the reaction of [Cu(tu)3]Cl with
In2(SO4)3 under refluxing conditions for a period of 3.5 h in ethyleneglycol.
Simulated PXRD pattern of WZ-CIS is provided for easy comparison. Insets show
(a) EDX spectrum with analysis, (b) SAED pattern of WZ-CIS, (c) room
temperature Raman spectrum obtained on excitation with 785 nm laser.
This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 18863–18871 | 18865
RSC Advances Paper

4. in the PXRD pattern of the reaction product of
[Cu2(tu)6]SO4?H2O and In(OAc)3 (Fig. 3(b)). In the room
temperature Raman spectrum for the CIS sample obtained
from the reaction between [Cu2(tu)6]SO4?H2O and In2(SO4)3,
bands at 305 and 345 cm21
have been observed suggesting it
to be only CIS, and negated the presence of any CuxS species
(due to the nonexistence of a band at around 470 cm21
) (inset
of Fig. 3).16
Curiously, the reaction of [Cu4(tu)9](NO3)4?4H2O
with In(OAc)3 and In2(SO4)3 independently yielded crystalline
CuS as deduced from their PXRD patterns (Fig. S5 ESI3). This
can possibly be due to a mismatch in the reactivity of
precursors in ethyleneglycol. Also, it appears that the genera-
tion of Cu9S5 (from other Cu-tu precursors) might be the prime
driving force for the formation of phases of pure WZ-CIS in
ethyleneglycol. Among the indium salts examined in the
present study, In2(SO4)3 would be an ideal starting material to
obtain ZB and WZ phases in ethyleneglycol. Both the WZ and
ZB-CIS obtained by the present procedure are redispersible in
non-polar solvents, such as hexane or toluene, which can
facilitate printing and spraying them on various substrates to
produce thin films.
It is relevant to compare our results with the report of Qi
et al.6a
wherein CuCl2?2H2O, InCl3?4H2O and thiourea have
been employed to synthesize metastable WZ-CIS solvother-
mally. Pure WZ-CIS has resulted from their reactions
conducted in strongly coordinating solvents such as ethano-
lamine.6a
Weaker coordinating solvents (such as DMF and
ethyleneglycol) yielded CH-CIS.6a
The reaction of [Cu(tu)3]Cl
with In2(SO4)3 in ethanolamine, after refluxing for 1.5 h, has
produced WZ-CIS suggesting the absence of a solvent role in
the selection of symmetry of the final product in this set of
reactions (Fig. S6 ESI3). Qi et al.6a
also varied the copper salts
from chloride to sulfate to nitrate, and observed the product to
be monophasic WZ-CIS with varying morphologies. However,
in the present set of reactions, [Cu(tu)3]Cl, [Cu2(tu)6]SO4?H2O
have yielded nanosized CIS (WZ/ZB) on refluxing with the
indium(III) salts with flower like morphology and the reactions
of [Cu4(tu)9](NO3)4?4H2O with the indium(III) salts have not
produced any ternary sulphide. All these observations suggest
a different mechanism for the formation of CIS in the present
set of reactions.
Wang et al.23
have reported the formation of CuIn12xGaxS2
possessing a wurtzite structure, with the use of a mixture of
strongly coordinating solvents and the appropriate copper,
indium, gallium and sulfur sources. The band gap increased
linearly from 1.5 eV (for CuInS2) to 2.48 eV for (CuGaS2).
Encouraged by the rapid and easy formation of WZ-CIS,
Fig. 2 (a) UV-Visible absorption spectrum of solid sample of WZ-CuInS2 (using
In2(SO4)3 as the indium source). Inset shows the band gap estimation plot. (b)
The photoluminescence spectrum of WZ-CIS on excitation with l = 500 nm.
Fig. 3 PXRD pattern of the products from the reaction of [Cu2(tu)6]SO4?H2O
with In2(SO4)3 and In(OAc)3?xH2O under refluxing conditions for a period of 2.5
h in ethyleneglycol. Simulated PXRD patterns of WZ and ZB-CIS are provided.
Inset shows the room temperature Raman spectrum of the product (In2(SO4)3 as
the indium source) obtained on excitation with 785 nm laser.
18866 | RSC Adv., 2013, 3, 18863–18871 This journal is ß The Royal Society of Chemistry 2013
Paper RSC Advances

5. reactions leading to compositions of the type CuIn12xGaxS2
have been examined. PXRD patterns of the products from the
reactions with the starting nominal compositions
CuIn12xGaxS2 (x = 0.25 and 0.50) after refluxing for 2.5 h are
shown in Fig. 4. The WZ phase has been found to be the major
phase with few reflections due to the CH-structure for the
reaction product with the nominal composition of
CuIn0.75Ga0.25S2. Existence of a hexagonal WZ phase (with
the diffracting spots corresponding to the 100, 102, 110
planes) along with the CH-phase (with diffracting spots
corresponding to the 020 plane) is also deduced from the
SAED pattern of these samples (inset of Fig. 4). The presence of
3.5 at% of gallium is detected from the EDX analysis of its FE-
SEM and HR-TEM images (inset of Fig. 4). Increasing the
refluxing time from 2.5 h to 30 h has not altered the amount of
indium to gallium in the CIS samples. Connor et al.24
have
also reached a similar conclusion in which strongly coordinat-
ing solvents have been employed to synthesize CuIn12xGaxS2.
Reflections due to WZ and CH phases are quite evident in the
PXRD pattern of the product from the reaction with a nominal
composition of CuIn0.5Ga0.5S2 (Fig. 4(b)). Ga3+
-doped WZ-CIS
samples have shown bands at 256, 288, 302 and 342 cm21
in
their room temperature Raman spectrum confirming the
predominantly hexagonal symmetry (inset of Fig. 4). An HR-
TEM image of gallium doped CIS with the EDX spectrum is
shown in Fig. 5 (a). A marginal increase in the absorption edge
(as compared to WZ-CIS) in the UV-visible spectrum has been
observed for the gallium doped samples (Fig. 5(b)). Reduced
transition intensity between the defect levels has been
observed in the PL spectrum on excitation with l = 500 nm
(inset of Fig. 5(b)).
CuInS2 has been examined as a host material for doping
magnetic as well as non-magnetic ions.24–26
While 20 at% of
Zn2+
-ion (non-magnetic) could be substituted for In3+
ion in
CuInS2 retaining the wurtzite structure, 15 at% of Fe3+
ion
could be doped in CIS.24,26
Very recently, Connor et al.24
have
extended the amount of iron in CuInS2 up to 20 at% by a
solution based synthetic strategy using oleate complexes of
copper, indium and iron in a mixture of strongly coordinating
solvents, dodecanethiol and oleylamine in which the coex-
istence of WZ and CH phases has been noticed for the higher
amounts of iron.24
Doping of Fe3+
-ion for In3+
-ion in WZ-CIS
by reacting [Cu(tu)3]Cl with In2(SO4)3 and Fe2(SO4)3 in
ethyleneglycol for 2.5 h has been examined. The symmetry of
the final products has shown dramatic changes as exemplified
by their PXRD patterns. While reflections typical of cubic-CIS
are observed in the PXRD pattern of the sample separated by
filtration under hot conditions (Fig. 6), tetragonal distortion
signifying the chalcopyrite type structure is quite evident in
the PXRD pattern of the sample filtered after natural cooling
(Fig. 6). From the Le Bail fitting, a cubic lattice constant of a =
Fig. 4 PXRD patterns of products from the reactions with the nominal
composition of (a) CuIn0.75Ga0.25S2 (b) CuIn0.50Ga0.50S2. Inset shows FE-SEM-
EDX analysis, SAED pattern and the Raman spectrum of gallium doped WZ-CIS
sample.
Fig. 5 (a) HR-TEM image of gallium doped WZ-CIS with the inset showing its
EDX spectrum, (b) Comparison of UV-Visible absorption spectrum of solid
sample of gallium doped CuInS2 (red filled circles) with undoped WZ-CIS in solid
form (black filled squares) with the inset showing the PL emission spectrum of
gallium doped CIS on excitation with l = 500 nm at room temperature.
This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 18863–18871 | 18867
RSC Advances Paper

6. 5.527(1) Å and tetragonal lattice constants of a = 5.522(4) Å and
c = 11.10(8) Å are obtained (Fig. S7, S8 ESI3). These lattice
constants are in conformity with the literature reports.24
The
room temperature Raman spectrum of iron doped CIS samples
is reproduced in the inset of Fig. 6 and shows four bands at
256, 297, 306 and 339 cm21
in the range 200–400 cm21
. The
band at 306 cm21
is higher in intensity than the one at 297
cm21
(usually the reverse is observed for pure CIS) suggesting
the presence of more Cu–Au ordering in them. The shift in the
position of the band from 344 to 339 cm21
indicates possible
internal changes in the dipole moments on the introduction of
Fe3+
ions into the system. In the FE-SEM image, again a flower
like morphology has been observed for the iron doped CIS
samples (Fig. 7 (a)). It appears that the products assume a
flower like morphology whenever thiourea has been employed
as the sulfur source in ethyleneglycol.16,14b,15a,15c
However, a
systematic study is warranted to correlate this proposition. An
HR-TEM image of iron doped CIS is shown in Fig. S9 (ESI3)
with the EDX analysis indicating the presence of 20.8 at% iron
(inset of Fig. 7 (b)). The tetragonal symmetry of iron doped
samples has also been confirmed from the SAED pattern
having spots corresponding to 112, 204, 116, 400 planes (Fig. 7
(b)). The transformation of hexagonal symmetry to cubic or
tetragonal on iron doping is quite similar to the change in
symmetry induced on Mn2+
(magnetic ion) doping in
hexagonal ZnS.27
The average crystallite size of iron doped
CIS in ZB and CH structures is y17 nm. The estimated band
gap value, from the UV-Visible diffuse reflectance spectrum,
has decreased from 1.40 eV (pure CIS) to 1.05 eV for the iron
doped samples (inset Fig. 8(a)). This is strong evidence for the
incorporation of Fe3+
in the samples since in such a case, it is
possible to introduce an intermediate band consisting of
delocalized d electrons26
(Fig. 8(a)). Fe3+
-containing CIS
samples have shown increased intensity of the PL emission
at 750 nm on excitation with 500 nm (inset of Fig. 8(a)). This
pointed to the presence of increased defect levels in these
samples. The presence of iron in the CIS samples has also
been confirmed from the atomic absorption spectroscopy and
EPR spectrum (inset of Fig. 8(b)). A broad signal at g = 1.90 has
been observed in the EPR spectrum confirming the incorpora-
tion of paramagnetic Fe3+
ions in the CIS lattice.28
The room
temperature magnetization measurements of Fe3+
-containing
CIS has indicated it to be paramagnetic with xg of 5.89 6 1026
emu g21
, differing from the diamagnetic behavior of the
undoped WZ-CIS (Fig. 8(b)). Thus, it can be concluded that
ethyleneglycol as solvent medium also promotes the facile
incorporation of Fe3+
-ion in the CIS lattice. At present, it is not
clear whether there exists a relation between the inclusion of
Fig. 6 PXRD pattern Fe3+
-doped CIS from the reaction of [Cu(tu)3]Cl with
In2(SO4)3?xH2O and Fe2(SO4)3?xH2O in ethyleneglycol for 2.5 h. Blue and red
plots indicate the PXRD patterns of the samples filtered under hot conditions
and after slow cooling. Inset shows the Raman spectrum of Fe3+
-doped CIS
obtained on excitation with l = 785 nm at room temperature.
Fig. 7 (a) FE-SEM image, (b) SAED pattern of CH-CIS sample containing iron.
Inset in (b) shows the EDX spectrum obtained from HR-TEM analysis.
18868 | RSC Adv., 2013, 3, 18863–18871 This journal is ß The Royal Society of Chemistry 2013
Paper RSC Advances

7. magnetic ions and transformation of cation arrangement in
CIS obtained by the solution based methodologies. Further
studies are required in this direction.
To verify the versatility of the current synthetic approach for
the generation of other metastable ternary metal sulfides,
synthesis of orthorhombic AgInS2 has been attempted using
[Ag2(tu)6]Cl2?2H2O and In(NO3)3 in ethyleneglycol. The pre-
sence of orthorhombic AgInS2 (JCPDS file no. 25-1328) is
evident from the PXRD pattern of the product after 1 h of
refluxing (Fig. 9). A few reflections pertaining to cubic AgIn5S8
(JCPDS file no. 25-1329), a known competing phase, are also
observed in the PXRD pattern.29
As AgInS2 samples are found
to be quite photosensitive, the Raman spectra have been
recorded using low laser powers (5–10% of 50 mW laser of l =
514 nm) in compact form. A sharp band at around 128 cm21
and a broad hump at around 290 cm21
have been observed for
AgInS2 matching closely with earlier reports29
(inset of Fig. 9).
4. Conclusions
A greener one pot solution process that is a rapid, easy to
handle, highly reproducible, synthetic process has been
developed for the production of metastable WZ-CIS starting
from cost effective copper-thiourea precursors and air stable
indium salts in the less coordinating solvent ethyleneglycol.
The experimental operation has been simplified without the
requirement of harsh conditions (high vacuum and high
temperature), thus reducing the cost and energy input
required for the scaling up process. The optical band gap,
estimated from UV-visible diffuse reflectance spectroscopy, is
1.40 eV. The Raman spectrum of CuInS2 has also confirmed
WZ to be the predominant crystalline phase. The generation of
copper rich sulfide from the dissociation of the copper-
thiourea precursor has been proposed to promote the
inclusion of In3+
ions to form a ternary phase in ethylenegly-
col. While smaller amounts of Ga3+
ions have been incorpo-
rated for In3+
ion in CIS samples retaining the WZ structure,
significant amounts of Fe3+
-ions have been included in CIS.
Iron containing CIS samples exhibit either a ZB or a CH
structure depending on the preparative conditions. A marginal
increase and a substantial decrease in the bandgap occurred
on the introduction of gallium and iron ions in CIS
respectively. Orthorhombic AgInS2 has also been obtained by
the rapid reaction of a silver-thiourea precursor with In(NO3)3
which validates the present methodology as a general
procedure for producing metastable ternary sulfides.
Fig. 8 (a) Comparison of UV-Visible absorption spectrum of Fe3+
-doped CuInS2
and undoped WZ-CIS in solid form. Insets show its band gap estimation plot and
the PL emission spectrum on exciting the sample with l = 500 nm (black filled
squares and blue filled circles represent CIS and Fe-doped CIS), (b) M–H curve for
the iron doped CIS sample with the inset showing its EPR spectrum at 298 K.
Fig. 9 PXRD pattern of AgInS2 from the reaction of [Ag2(tu)6]Cl2 with
In(NO3)3?xH2O in ethyleneglycol after a refluxing period of 1 h (reflections
marked with * are due to AgIn5S8, JCPDS file no. 25-1329). Inset shows its
Raman spectrum excited with l = 514 nm at room temperature.
This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 18863–18871 | 18869
RSC Advances Paper

8. Acknowledgements
Authors wish to record their sincere thanks to DST
(Nanomission) for funding this research. MG and PK thank
CSIR, New Delhi and DST respectively for their fellowship.
Useful discussions with Dr S. Uma, Department of Chemistry,
University of Delhi, Delhi are gratefully acknowledged. Also,
thanks are due to the University of Delhi, Delhi for the usage of
facilities of USIC, M. Tech (NSNT) program. Authors thank Ms
Prachi Agrawal for help in some aspects of synthesis.
References
1 (a) M. Gratzel, Philos. Trans. R. Soc. London, Ser. A, 2007,
365, 993–1005; (b) T. K. Ahn, T. J. Avenson, M. Ballottari, Y.-
C. Cheng, K. K. Niyogi, R. Bassi and G. R. Fleming, Science,
2008, 320, 794–797; (c) W. Morales, M. Cason, O. Aina, N.
R. Tacconi and K. Rajeshwar, J. Am. Chem. Soc., 2008, 130,
6318–6319; (d) L. Zheng, Y. Xu, Y. Song, C. Wu, M. Zhang
and Y. Xie, Inorg. Chem., 2009, 48, 4003–4009.
2 (a) R. W. Birkmire and E. Eser, Annu. Rev. Mater. Sci., 1997,
27, 625–653; (b) M. I. Schimmel, O. L. Bottechia and
H. Wendt, J. Appl. Electrochem., 1998, 28, 299–304; (c) G.
C. Park, H. D. Chung, C. D. Kim, H. R. Park, W. J. Jeong, J.
U. Kim, H. B. Gu and K. S. Lee, Sol. Energy Mater. Sol. Cells,
1997, 49, 365–374; (d) T. Watanabe and M. Matsui, Jpn. J.
Appl. Phys., 1996, 35, 1681–1684.
3 (a) D. C. Look and J. C. Manthuruthil, J. Phys. Chem. Solids, 1976,
37, 173–180; (b) K. J. Backman, M. Fearheiley, Y. H. Shing and
N. Tran, Appl. Phys. Lett., 1984, 44, 407–409; (c) D. Cahen, in
Ternary and multinary compounds, ed. S. K. Deb and A. Zunger,
Materials Research Society, Pittsburgh, 1987, p. 443; D. Y. Chang,
H. Y. Ueng, Proceedings of the 9th International Conference on
Ternary and Multinary Compounds, 1993, p. 198; (d) F. M. Courtel,
A. Hammami, R. Imbeault, G. Hersant, R. W. Paynter, B. Marsan
and M. Morin, Chem. Mater., 2010, 22, 3752–3761; (e) G. Morell,
R. S. Katiyar, S. Z. Weisz, T. Walter, H. W. Schock and I. Balberg,
Appl. Phys. Lett., 1996, 69, 987–989.
4 J. J. M. Binsma, L. J. Giling and J. Bloem, J. Cryst. Growth,
1980, 50, 429–436.
5 (a) K. Nose, Y. Soma, T. Omata and S. Otsuka-Yao-Matsuo, Chem.
Mater., 2009, 21, 2607–2613; (b) M. E. Norako, M. A. Franzman
and R. L. Brutchey, Chem. Mater., 2009, 21, 4299–4304; (c) P. Bera
and S. Seok, J. Solid State Chem., 2010, 183, 1872–1877; (d) N. Bao,
X. Qiu, Y.-H. A. Wang, Z. Zhou, X. Lu, C. A. Grimes and A. Gupta,
Chem. Commun., 2011, 47, 9441–9443.
6 (a) Y. Qi, Q. Liu, K. Tang, Z. Liang, Z. Ren and X. Liu, J.
Phys. Chem. C, 2009, 113, 3939–3944; (b) M. Kruszynska,
H. Borchert, J. Parisi and J. Koiny-Olesiak, J. Am. Chem.
Soc., 2010, 132, 15976–15986; (c) X. Lu, Z. Zhuang, Q. Peng
and Y. Li, CrystEngComm, 2011, 13, 4039–4045.
7 (a) B. Koo, R. N. Patel and B. A. Korgel, Chem. Mater., 2009,
21, 1962–1966; (b) S. T. Connor, C.-M. Hsu, B. D. Weil,
S. Aloni and Y. Cui, J. Am. Chem. Soc., 2009, 131, 4962–4966;
(c) S.-H. Choi, E.-G. Kim and T. Hyeon, J. Am. Chem. Soc.,
2006, 128, 2520–2521.
8 (a) A. L. Abdelhady, M. A. Malik and P. O’Brien, J. Mater.
Chem., 2012, 22, 3781–3785; (b) W. Han, L. Yi, N. Zhao,
A. Tang, M. Gao and Z. Tang, J. Am. Chem. Soc., 2008, 130,
13152–13161.
9 (a) C. C. Landry, J. Lockwood and A. R. Barron, Chem.
Mater., 1995, 7, 699–706; (b) L. Shi, C. Pei and Q. Li,
Nanoscale, 2010, 2, 2126–2130; (c) M. G. Panthani,
V. Akhavan, B. Goodfellow, J. P. Schmidtke, L. Dunn,
A. Dodabalapur, P. F. Barbara and B. A. Korgel, J. Am.
Chem. Soc., 2008, 130, 16770–16777; (d) K. Yoshino,
T. Ikari, S. Shirakata, H. Miyake and K. Hiramatsu, Appl.
Phys. Lett., 2001, 78, 742–744; (e) C. J. Carmalt, D.
E. Morrison and I. P. Parkin, J. Mater. Chem., 1998, 8,
2209–2211.
10 (a) Q. Lu, J. Hu, K. Tang, Y. Qian, G. Zhou and X. Liu, Inorg.
Chem., 2000, 39, 1606–1607; (b) J. Xiao, Y. Xie, R. Tang and
Y. Qian, J. Solid State Chem., 2001, 161, 179–183; (c) J. Xiao,
Y. Xie, Y. Xiong, R. Tang and Y. Qian, J. Mater. Chem., 2001,
11, 1417–1420; (d) S. Liu, H. Zhang, Y. Qiao and X. Su, RSC
Adv., 2012, 2, 819–825.
11 (a) H. Nakamura, W. Kato, M. Uehara, K. Nose, T. Omata,
S. Ostsuka-Yao-Matsuo, M. Miyazaki and H. Maeda, Chem.
Mater., 2006, 18, 3330–3335; (b) J.-Y. Chang and C.-Y. Cheng,
Chem. Commun., 2011, 47, 9089–9091; (c) A. Pein,
M. Baghbanzadeh, T. Rath, W. Haas, E. Maier, H. Amenitsch,
F. Hofer, C. O. Kappe and G. Trimmel, Inorg. Chem., 2011, 50,
193–200.
12 (a) D. Pan, L. An, Z. Sun, W. Hou, Y. Yang, Z. Yang and
Y. Lu, J. Am. Chem. Soc., 2008, 130, 5620–5621; (b) S.
K. Batabyal, L. Tain, N. Venkatram, W. Ji and J. J. Vittal, J.
Phys. Chem. C, 2009, 113, 15037–15042; (c) K.-T. Kuo, S.-
H. Hu, D.-M. Liu and S.-Y. Chen, J. Mater. Chem., 2010, 20,
1744–1750.
13 (a) S. L. Castro, S. G. Bailey, R. P. Raffaelle, K. K. Banger and A.
F. Hepp, J. Phys. Chem. B, 2004, 108, 12429–12435; (b) H. Zhong,
Y. Zhou, M. Ye, Y. He, J. Ye, C. He, C. Yang and Y. Li, Chem.
Mater., 2008, 20, 6434–6443; (c) J. S. Gardner, E. Shurdha,
C. Wang, L. D. Lau, R. G. Rodriguez and J. J. Pak, J. Nanopart. Res.,
2008, 10, 633–641; (d) T. C. Deivaraj, J.-H. Park, M. Afzaal,
P. O’Brien and J. J. Vittal, Chem. Mater., 2003, 15, 2383–2391.
14 (a) D.-G. Lee, N.-H. Lee, H.-J. Oh, S.-C. Jung, J.-S. Hwang, W.-
J. Lee and S.-J. Kim, J. Nanosci. Nanotechnol., 2011, 11,
1434–1437; (b) F. Long, W.-M. Wang, H.-C. Tao, T.-K. Jia, X.-
M. Li, Z.-G. Zou and Z.-Y. Fu, Mater. Lett., 2010, 64, 195–198;
(c) M. Tang, Q. Tian, X. Hu, Y. Peng, Y. Xue, Z. Chen, J. Yang,
X. Xu and J. Hu, CrystEngComm, 2012, 14, 1825–1832; (d) D.
P. Dutta and G. Sharma, Mater. Lett., 2006, 60, 2395–2398.
15 (a) A. Zhang, Q. Ma, M. Lu, G. Yu, Y. Zhou and Z. Qiu, Cryst.
Growth Des., 2008, 8, 2402–2407; (b) Y. Akaki, T. Matsubara,
Y. Ohno, T. Momiki and K. Ide, Phys. Status Solidi C, 2009,
6, 1027–1029; (c) H. Hu, B. Yang, X. Liu, R. Zhang and
Y. Qian, Inorg. Chem. Commun., 2004, 7, 563–565; (d)
G. Shen, D. Chen, K. Tang, Z. Fang, J. Sheng and Y. Qian, J.
Cryst. Growth, 2003, 254, 75–79.
16 P. Kumar, M. Gusain and R. Nagarajan, Inorg. Chem., 2011,
50, 3065–3070.
17 A. Neisser, I. Hengel, R. Klenk, T. W. Matthes, J. Alvarez-
Garcia, A. Perez-Rodriguez, A. Romano-Rodriguez and M.
C. Lux-Steiner, Sol. Energy Mater. Sol. Cells, 2001, 67,
97–104.
18 R. D. Shannon and C. T. Prewitt, Acta Crystallogr., Sect. B:
Struct. Crystallogr. Cryst. Chem., 1969, 25, 925–941.
19 P. Bombicz, I. Mutikainen, M. Krunks, T. Leskela, J. Madarasz
and L. Niinisto, Inorg. Chim. Acta, 2004, 375, 513–525.
18870 | RSC Adv., 2013, 3, 18863–18871 This journal is ß The Royal Society of Chemistry 2013
Paper RSC Advances

9. 20 G. A. Bowmaker, C. Pakawatchai, S. Saithong, B. W. Skelton
and A. H. White, Dalton Trans., 2010, 39, 4391–4404.
21 B. Tell, J. L. Shay and H. M. Kasper, Phys. Rev. B: Solid State,
1972, 4, 2463–2471.
22 M. Uehara, K. Watanabe, Y. Tajiri, H. Nakamura and
H. Maeda, J. Chem. Phys., 2008, 129, 134709–134714.
23 Y.-H. A. Wang, X. Zhang, N. Bao, B. Lin and A. Gupta, J. Am.
Chem. Soc., 2011, 133, 11072–11075.
24 S. T. Connor, B. D. Weil, S. Misra, Y. Cui and M. F. Toney,
Chem. Mater., 2013, 25, 320–325.
25 (a) N. Tsujii, H. Kitazawa and G. Kido, Phys. Status Solidi A,
2002, 3, 951–954; (b) A. Pietnoczka, R. Bacewicz and
S. Schorr, Phys. Status Solidi A, 2006, 203, 2746–2750.
26 J. D. Burnett, T. Xu, M. Sorescu, B. R. Strohmeier, J. Sturgeon,
O. Gourdon, K. Baroudi, J.-L. Yao and J. A. Aitken, J. Solid State
Chem., 2013, 197, 279–287.
27 (a) S. Biswas, S. Kar and S. Chaudhuri, J. Phys. Chem. B,
2005, 109, 17526–17530; (b) S. Biswas and S. Kar,
Nanotechnology, 2008, 19, 045710.
28 G. P. Bernardini, D. Borrini, A. Caneschi, F. Di Benedetto,
D. Gatteschi, S. Ristori and M. Romanelli, Phys. Chem.
Miner., 2000, 27, 453–461.
29 (a) T. Ogawa, T. Kuzuya, Y. Hamanaka and K. Sumiyama, J.
Mater. Chem., 2010, 20, 2226–2231; (b) S. P. Hong, H.
K. Park, J. H. Oh, H. Yang and Y. R. Do, J. Mater. Chem.,
2012, 22, 18939–18949; (c) M. Ortega-Lopez, O. Vigil-Galan,
F. C. Gandarilla and O. Solorza-Feria, Mater. Res. Bull.,
2003, 38, 55–61.
This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 18863–18871 | 18871
RSC Advances Paper